posix_fallocate - allocate file space
#include <fcntl.h>
int posix_fallocate(int fd, off_t offset, off_t len);
Feature Test Macro Requirements for glibc (see
feature_test_macros(7)):
posix_fallocate():
_POSIX_C_SOURCE >= 200112L
The function
posix_fallocate() ensures that disk space is allocated for
the file referred to by the file descriptor
fd for the bytes in the
range starting at
offset and continuing for
len bytes. After a
successful call to
posix_fallocate(), subsequent writes to bytes in the
specified range are guaranteed not to fail because of lack of disk space.
If the size of the file is less than
offset+
len, then the file is
increased to this size; otherwise the file size is left unchanged.
posix_fallocate() returns zero on success, or an error number on failure.
Note that
errno is not set.
- EBADF
- fd is not a valid file descriptor, or is not opened for
writing.
- EFBIG
- offset+len exceeds the maximum file size.
- EINTR
- A signal was caught during execution.
- EINVAL
- offset was less than 0, or len was less than or equal to 0,
or the underlying filesystem does not support the operation.
- ENODEV
- fd does not refer to a regular file.
- ENOSPC
- There is not enough space left on the device containing the file referred
to by fd.
- ESPIPE
- fd refers to a pipe.
posix_fallocate() is available since glibc 2.1.94.
For an explanation of the terms used in this section, see
attributes(7).
| Interface |
Attribute |
Value |
| posix_fallocate () |
Thread safety |
MT-Safe (but see NOTES) |
POSIX.1-2001.
POSIX.1-2008 says that an implementation
shall give the
EINVAL
error if
len was 0, or
offset was less than 0. POSIX.1-2001 says
that an implementation
shall give the
EINVAL error if
len
is less than 0, or
offset was less than 0, and
may give the
error if
len equals zero.
In the glibc implementation,
posix_fallocate() is implemented using the
fallocate(2) system call, which is MT-safe. If the underlying
filesystem does not support
fallocate(2), then the operation is
emulated with the following caveats:
- *
- The emulation is inefficient.
- *
- There is a race condition where concurrent writes from another thread or
process could be overwritten with null bytes.
- *
- There is a race condition where concurrent file size increases by another
thread or process could result in a file whose size is smaller than
expected.
- *
- If fd has been opened with the O_APPEND or O_WRONLY
flags, the function fails with the error EBADF.
In general, the emulation is not MT-safe. On Linux, applications may use
fallocate(2) if they cannot tolerate the emulation caveats. In general,
this is only recommended if the application plans to terminate the operation
if
EOPNOTSUPP is returned, otherwise the application itself will need
to implement a fallback with all the same problems as the emulation provided
by glibc.
fallocate(1),
fallocate(2),
lseek(2),
posix_fadvise(2)